Fix clang undefined behavior sanitizer errors.

Signed and unsigned integer overflows, no actual consequences
but clean up the errors.

Backport of commits 6801500e23 and 69e02afb6e.

pb_common.c

@@ -11,8 +11,17 @@ bool pb_field_iter_begin(pb_field_iter_t *iter, const pb_field_t *fields, void *
     iter->pos = fields;
     iter->required_field_index = 0;
     iter->dest_struct = dest_struct;
-    iter->pData = (char*)dest_struct + iter->pos->data_offset;
-    iter->pSize = (char*)iter->pData + iter->pos->size_offset;
+
+    if (!dest_struct)
+    {
+        iter->pData = NULL;
+        iter->pSize = NULL;
+    }
+    else
+    {
+        iter->pData = (char*)dest_struct + iter->pos->data_offset;
+        iter->pSize = (char*)iter->pData + iter->pos->size_offset;
+    }
     
     return (iter->pos->tag != 0);
 }

pb_decode.c

@@ -1222,7 +1222,7 @@ static void pb_release_single_field(const pb_field_iter_t *iter)
         
         if (pItem)
         {
-            while (count--)
+            for (; count > 0; count--)
             {
                 pb_release((const pb_field_t*)iter->pos->ptr, pItem);
                 pItem = (char*)pItem + iter->pos->data_size;
@@ -1239,7 +1239,7 @@ static void pb_release_single_field(const pb_field_iter_t *iter)
             /* Release entries in repeated string or bytes array */
             void **pItem = *(void***)iter->pData;
             pb_size_t count = *(pb_size_t*)iter->pSize;
-            while (count--)
+            for (; count > 0; count--)
             {
                 pb_free(*pItem);
                 *pItem++ = NULL;

pb_encode.c

@@ -852,7 +852,6 @@ static bool checkreturn pb_enc_fixed32(pb_ostream_t *stream, const pb_field_t *f
 static bool checkreturn pb_enc_bytes(pb_ostream_t *stream, const pb_field_t *field, const void *src)
 {
     const pb_bytes_array_t *bytes = NULL;
-    size_t allocsize;
 
     bytes = (const pb_bytes_array_t*)src;
     
@@ -862,9 +861,8 @@ static bool checkreturn pb_enc_bytes(pb_ostream_t *stream, const pb_field_t *fie
         return pb_encode_string(stream, NULL, 0);
     }
     
-    allocsize = PB_BYTES_ARRAY_T_ALLOCSIZE(bytes->size);
-    if (allocsize < bytes->size ||
-        (PB_ATYPE(field->type) == PB_ATYPE_STATIC && allocsize > field->data_size))
+    if (PB_ATYPE(field->type) == PB_ATYPE_STATIC &&
+        bytes->size > field->data_size - offsetof(pb_bytes_array_t, bytes))
     {
         PB_RETURN_ERROR(stream, "bytes size exceeded");
     }